If the diverse and numerous dinosaurs (except birds) are extinct, how can we better understand how
they lived? Even though the great dinosaurs of the
Mesozoic are gone, they have left us
many clues. Dinosaur fossils are not limited to bones, but include skin, eggs, nests,
footprints, and other special kinds of fossils that give us clues about
their lifestyles.

Dinosaur nests

The news has recently carried several stories of wonderful finds of nesting
dinosaurs, and it is true that an explosion of data on dinosaurian nesting and
social behavior has been uncovered in the past 20 years. Some of the most well
known and compelling evidence comes from Jack Horner's
(Museum of the Rockies) ongoing work at the "Egg Mountain" site in Montana,
where he has documented evidence of a large nesting area used by
hadrosaurian
(duckbilled) dinosaurs. These dinosaurs were named Maiasaura, "good mother
reptile," referring to the closely packed nests that contain fossilized
eggs, embryos, and juveniles (such as the one pictured at right).
This is one case where we can be fairly confident that
parental care was involved in these dinosaurs' lifestyle. Actually, this is
not a surprising assertion, because both crocodilians (their closest living
relatives) and birds
(their living descendants), both show some degree of
parental care and extensive nest building.

Other dramatic finds of dinosaur nests include
theropod dinosaurs (Oviraptor and Troodon) that
apparently died while brooding their nests, and abundant nests of the early
ceratopsian
dinosaur Protoceratops. An interesting story about Oviraptor:
the so-called "egg stealer" was so named because it was found atop a
clutch of eggs that were assumed to belong to Protoceratops. This idea
held for some 70 years until a find in the 1990s showed an Oviraptor
embryo inside one of those eggs..."egg stealer" exonerated!

Dinosaur footprints

We know of literally thousands of non-avian dinosaur footprints
scattered around the globe, from
Late Triassic to Late Cretaceous age. You might not think that a footprint or
a sequence of footprints (called a trackway) could tell us much, but actually
it can tell us some general things about the biology of dinosaurs.

From trackway data, we can tell that:

Some non-avian dinosaurs travelled in large groups;

Non-avian dinosaurs moved with their feet held underneath their body (as birds
and mammals do); and

Some non-avian dinosaurs moved rather quickly,
but some plodded along at a more leisurely pace  see our section on
dinosaur speeds for more info.

Dinosaur diet

Dinosaurs, living and extinct, have varied diets. We have some strong evidence
of exactly what the diets of some of the extinct dinosaurs was, and we can
observe birds directly to learn about their diets. Dentition (tooth
structure) is one
of the most abundant lines of evidence useful for determining dinosaur diets.
Most ornithischian
and sauropodomorph dinosaurs had rather simple, short stubby
crenellated teeth, which are similar to those of living herbivores, and clearly
not too good for eating much meat.

Theropod teeth, on the other hand, retain
the primitive archosaurian characteristic of being recurved, serrated, laterally
-compressed, and knife-like. There is some variation in tooth structure among
extinct theropods, but most are fairly similar and obviously related to a
carnivorous diet.

Stomach contents are another line of evidence, somewhat more direct but also
a bit trickier to interpret accurately. Well-preserved dinosaur skeletons
sometimes have traces of apparent food items preserved in their abdominal
cavity, where it's safe to assume that they had a stomach. This includes pine
cones and/or needles in some herbivores' guts, and traces of some vertebrates
in some theropods' guts. So this independent line of inquiry substantiates the
data from tooth morphology. Also, some sauropodomorph stomachs contain well-
rounded stones, caled gastroliths, that were probably used to grind food in
a muscular crop or gizzard, like some birds (and crocodilians) do.

The general hypothesis that most ornithischians and sauropodomorphs were
largely, if not completely herbivorous, and that theropods (at least before
the origin of birds) were mostly carnivorous, thus holds. More specific
hypotheses have been proposed and supported by data, while others have fallen
by the wayside. It is likely that new discoveries will illuminate more about
dinosaur diets as the global "dinosaur renaissance" continues.